This invention relates to battery state of charge monitoring systems, and more particularly, to a method and the apparatus for determining battery state of charge by measuring the phase angle change of an A.C. voltage applied to the battery.
There are many applications where it is essential to know the state of charge of batteries. For example, in an electrical vehicle, measuring the state of charge of the battery or batteries utilized in the vehicle permits one to determine the useful life remaining before the battery or batteries need to be recharged and permits one to charge the battery or batteries without overcharging. There are, of course, many other applications where it is essential to know the state of charge of the batteries being utilized.
There are, of course, several different methods known in the art for determining the state of charge of a cell or battery of cells. One such prior art method utilizes current integrators. When utilizing these current integrators one must correct the measurement obtained since several factors affect the accuracy of this measurement. Some factors that affect the measurement of state of charge current integrators are: temperature, previous history, efficiency and cycle life.
Another method commonly used is the measurement of the specific gravity of the battery. Specific gravity measurements are not accurate, require long stand times for equilibrium and do not provide proportional voltage read outs. Further, for some batteries, such as nickel cadmium batteries, specific gravity measurements do not provide a state of charge indication since the specific gravity of a nickel cadmium battery does not change. In addition, specific gravity measurements of sealed batteries can obviously not be obtained.
Still another prior art method used to measure battery state of charge involves measuring state of charge with battery response to current or voltage pulses. These pulse systems are go-no-go systems that provide a rough means of relating state of charge and are used primarily to determine if a battery is still useful.
Another known method of measuring state of charge involves impedance measurements utilizing bridge circuits that are manually adjusted. The measurements obtained are compared with previously generated standard reference curves. These bridge systems are difficult to automate.
This invention provides a means of continuously following a cell or battery state of charge by monitoring the phase change of an A.C. voltage applied to the cell or battery being monitored. Electronic circuitry is utilized; therefore, the system operates automatically and the measurements obtained need not be corrected for various factors that affect the accuracy of the measurements obtained by some of the prior art systems.